Hydraulic fracturing is used to create cracks in oil- or gas-bearing formations to facilitate movement of oil or gas to a well bore and stimulate production from that well.
Fracturing is performed by pumping a fluid at high pressure down the well and into the formation. The fluid may be water, carbon dioxide or nitrogen, for example. The fluid usually contains sand or other “proppant” to hold the fracture open after the fracturing fluid has dissipated or been pumped out, as well as viscosity modifiers and other optional components.
Immediately after a gas well has been stimulated, for example using nitrogen as the fracturing fluid, the nitrogen content of the gas emerging from the well is extremely high. After a short time, typically no more than a week or ten days, the gas returns to its pre-stimulation composition. The same situation occurs with any other fracturing gas, such as carbon dioxide.
Until the fracturing gas content drops, the gas is usually unsuitable to be sent to its original destination, typically a gas pipeline or gas processing facility. Absent any technically or economically feasible treatment technology, the stimulant-rich gas is simply vented or flared, options that waste otherwise valuable natural gas that is coproduced during this period and are environmentally harmful.
Technologies for removing nitrogen from natural gas are known, specifically cryogenic distillation and membrane separation. Unless the well happens to be adjacent to a gas processing plant with cryogenic distillation capabilities, it is clearly not practical to apply cryogenic technology for such a short-term use. In addition, the rapidly changing nitrogen content of the gas would make the use of cryogenics very difficult.
Membrane-based gas separation systems are robust, modular, skid-mounted units that are much more amenable to being incorporated into a movable system than large cryogenic distillation columns. U.S. Pat. No. 6,955,704, to Strahan, describes the use of a mobile unit to treat gas immediately after a well has been stimulated using carbon dioxide. The unit includes a pretreatment system to remove solids, hydrocarbons and water, and a membrane unit to separate carbon dioxide from the raw gas.
Even a membrane system will have difficulties treating post-stimulation gas, however, because the performance of the membrane unit varies with the feed gas composition, feed flow rate and gas pressure. For this reason, membrane systems are not used to treat unstable gas streams characterized by substantially changing parameters. In the case of a gas stream produced by a well after stimulation, the difficulty is exacerbated because the concentration of fracturing gas changes by the hour, especially in the first days after stimulation. The flow rate of the gas also varies rapidly, peaking within a few hours and subsequently decreasing progressively.
As the content of fracturing gas in the produced gas drops, the compositions of the residue and permeate streams from a membrane unit will change substantially. A system designed to treat the initial gas composition will significantly overprocess the gas as time goes on, wasting energy to do so. The product streams may also fail to meet the target specifications for their pre-stimulation destinations.
Likewise, a system designed for a certain flow rate or feed pressure will not be able to maintain consistent performance if the flow or pressure of the feed gas increases or decreases.
In light of these problems, the need for treatment that is both mobile and able to process gas of changing composition or flow rate remains.